1. Field
This invention relates generally to a multichannel laser amplifier array including polarization control for each beam channel to provide co-polarization in a coherently combined beam and, more particularly, to a multichannel laser amplifier array including a single polarization detector that detects the polarization for each beam channel after the beams are combined and a polarization demultiplexer for each beam channel that measures a phase dither in each beam to provide co-polarization in the coherently combined beam.
2. Discussion
High power laser amplifiers have many applications, including industrial, commercial, military, etc. Designers of laser amplifiers are continuously investigating ways to increase the power of the laser amplifier for these applications. One known type of laser amplifier is a multichannel high power fiber laser amplifier that employs doped fibers and pump beams to generate the laser beam. Typically, a high power fiber laser amplifier uses a fiber that has an active core diameter of about 10-20 μm or larger. Modern fiber laser amplifier designs have achieved single fiber power levels up to 10 kW.
A design challenge for multichannel fiber laser amplifiers is to combine the beams from each fiber in a coherent manner so that the beams provide a single output beam having a uniform phase over the beam diameter such that the beam can be focused to a small focal spot. Focusing the combined beam to a small spot at a long distance (far-field) defines the beam quality of the beam, where the more coherent the individual fiber beams the more uniform the combined phase and the better the beam quality. Improvements in fiber laser amplifier designs increase the output power and coherency of the fiber beams in such a way as to approach the theoretical power and beam quality limit of the laser system.
The fiber beams in each channel of a multichannel fiber laser power amplifier array must also be co-polarized in the coherently combined beam to achieve high beam power and efficiency. Any depolarized light in the fiber beams will not combine coherently with the other beams and is effectively lost. The fibers comprising the fiber amplifier array induce depolarization in the fiber beams, which limits the combining efficiency of the fiber beams. It is known in the art to measure or detect the polarized power of each fiber beam in the amplifier array after the beam has been amplified and use the measured polarized power as an optimization control metric to provide feedback in a feedback loop to a polarization device to adjust the polarization of the beam to a desired polarization for all of the fiber beams. For a laser amplifier array including many fiber beams, for example, several hundred fiber beams, that are combined into a single combined beam, it is typically opto-mechanically complex to implement a large number of polarization sensors at the output of each fiber amplifier, where a separate sensor would be required for each fiber beam.